The chemist in me says that the corn (Karo) syrup is a bit of insurance in keeping the toffee from being rock-hard when you're done. I went to make our toffee this year and was out of Karo/King syrup, so substituted with agave nectar and didn't see any difference. For whatever reason, recipes that add Karo also add some water (or rum!) which is going to cook away.
Another trick is to add a bit (maybe 1/2 tsp) of baking soda to the toffee at the very end when stirring in the nuts; in larger doses, adding baking soda is what makes brickle instead of toffee, creating some small air bubbles so the product is not so hard.
This recipe looks not too different from the toffee part of what I do, and seems to be what you're looking for:
http://www.landolakes.com/recipe/2708/b ... ter-toffee. And of course to be trendy you can toss a bit of sea salt on the chocolate at the end!
The recipe I use pours on melted chocolate and spreads it before adding (more) nuts; then the whole thing gets flipped over and coated with chocolate and nuts on the other side. Yum. Using two sheets of Silpat and two baking sheets takes the scary part of the work away. (I put one sheet of Silpat on a baking sheet, pour the hot toffee on that and spread as best as you can, then cover that with another Silpat and baking sheet to smooth it out evenly. Take off the upper sheet/Silpat to spread the first layer of chocolate and nuts. Let that cool, then put the Silpat/baking sheet back on to flip the whole thing over.) We got this recipe from a friend who uses almonds; to make ours different we usually use pistachios (and add a pinch of cayenne pepper to the melted chocolate).
David
....I do know it had butter, sugar, karo sryup.....you used a candy thermometer, then remove from stove, add vanilla, pour out on a greased 9X13 pan...then you would sprinkle a bag of semi-chocolate on top, wait a couple minutes and then spread.....then add your walnuts on the top....
.....I have googled and see them like mine in a way...but it really kills me that I can't find MINE! LOL....that's what I get for being so dis-organized....I will just have to pick one, and make it!! 
Thanks!!
